State-of-the-art processes to fabricate thermoplastic high temperature polymer/graphite composites includes a pretreatment imidization process for removing volatiles by advancing the resin/composite of laid-up uncured plies from room temperature to 200.degree. C. before the application of pressure. This portion of the fabrication process is carried out independent of the consolidation of the uncured plies, typically in an autoclave or compression molding process. From room temperature to 200.degree. C., most of the gas releasing reactions which are required for imidization and/or chain extension have occurred, and it is assumed that further processing at elevated temperatures serves primarily to consolidate the resin/fiber system, with a minimal amount of cure. This minimal amount of cure is assumed to have a benign affect on the fabrication process.
A typical pretreatment process consists of drying and imidizing freshly made prepreg in an oven cycle at 93.degree. C. (200.degree. F.) for one hour, 149.degree. C. (300.degree. F.) for an additional hour, and 204.degree. C. (400.degree. F.) for another two hours. The material is said to be capable of being put through the final stages of cure with no further evolution of volatiles, starting, if desired, with a press and/or mold preheated to 221.degree. C. (430.degree. F.).
Experience has shown, however, that composites fabricated in this manner can often range from low to high void and are more prone to blistering. These properties are undesirable because they tend to detract from the physical properties and thermal oxidative stability characteristics of these polymers. As such, overall yield and long term reliability of such composite systems is reduced as composite fabrication costs increase.
Thus, a long felt need exists to convert polyimide systems into stable materials by pretreating such systems in a process which prevents evolution of volatile gas products on further processing at the higher temperatures required for consolidation and final cure of such systems.